Survey lines revisited: finding nonundercut surfaces for retainers that approach from the occlusal surface

This study investigated survey lines on abutments in the partially edentulous mouth. Various methods that were used to determine the path of insertion had little impact on the position of survey lines except on tooth surfaces with minimal undercuts. Typical survey lines are described and according to the survey lines, few tooth surfaces could accommodate the rigid portion of an occlusally approaching retainer and permit it to be positioned on a nonundercut tooth surface. Although some degree of enameloplasty to improve the situation on some tooth surfaces is possible, the degree to which this can be done is limited by the thickness of the enamel and considerations of tooth conservation.